The present invention relates to an improved racket frame and a method for manufacturing same, and more particularly relates to improvement in the foamed resin core construction of a fiber reinforced plastic racket frame made by molding process and advantageously usable for tennis rackets.
In the conventional method for manufacturing a fiber reinforced racket frame, an elongated foamed resin core of a longitudinally continuous construction is wholly wrapped and covered by a fiber reinforced plastic envelope which contains numerous reinforcing fibers preimpregnated with thermosetting resin. The foamed resin core functions as a member for damping elastic vibratory waves generated by impact imposed on the playing face of the racket at striking balls and the fiber reinforced envelope functions as a member for bearing the impulsive stresses working on the racket at striking balls.
Thus, in the conventional racket frame, the foamed resin core is quite continuous over the entire length thereof and the transverse cross-sectional characteristics and the density are both substantially over the entire length thereof. From the viewpoint of vibration system, the above-described continuous construction and mechanical uniformity of the conventional racket frame are well suited for smooth transmission of elastic vibratory waves.
Therefore, with the above-described core construction in the conventional racket frame, vibrations generated by impact imposed on the strings at striking balls are transmitted quite directly and smoothly to players' hand via the throat and shaft portions of the racket frame and give the players "hard touch" at striking balls and the so-called "tennis elbow" troubles. In addition, the uniformity in the transverse cross-sectional characteristics and density results in uniform distribution of weight in various portions of the racket frame. This uniform weight distribution causes difficulty in delicate control in swinging the racket at striking balls.
In order to obviate these disadvantages, it is proposed to change the geometrical moment of inertia from portion to portion along the length of the racket frame. This can be attained by changing the transverse cross-sectional characteristics, such as the transverse cross sectional surface area and profile, from portion to portion of the racket frame. This change in the geometrical moment of inertia may successfully hinder smooth transmission of the elastic vibratory waves.
In practical production, the above-described foamed resin cores are formed by cutting a longer material rod into several blocks of a prescribed length and, for advantage in productivity, the elongated material rod is formed by continuous extrusion of the material resin. With this extrusion system, it is almost improssible in practice to provide the extruded material rod with periodical local change in the transverse cross-sectional characteristics, i.e. the geometrical moment of inertia. So, in order to change the transverse cross-sectional characteristics from portion to portion of a racket frame, it is necessary to apply mechanical surface cutting to individual cores after cutting from the extruded longer material rod. Such individual handling system is clearly unsuited for process automatization and mass-production, which, as well known, assure high productivity with reduced labour and, therefore, advantage in economy.